No. 3.] THE TAIL IN LUMBRICULUS. 329 



tribution. They begin to develop at an appreciable time after 

 the circular muscles are formed. 



In the meantime the cavity in the centre of the median meso- 

 dermal element has grown larger, and the cells have arranged 

 themselves around it as a wall. The dorsal part of this wall 

 now bends down ventrally, forming a groove. The sides of the 

 groove close in above, and it becomes a tube, which hangs sus- 

 pended in the cavity of the former median element. This tube 

 is the ventral blood-vessel. The part of the wall that does not 

 take part in the formation of the ventral blood-vessel forms 

 the ventral mesentery by which the blood-vessel is suspended 

 (Fig. II). 



PART III. GENERAL. 

 V. Regeneration and Agamic Reproduction. 



The general aspects of the preceding work are to be sought 

 in the relation of regeneration to agamic reproduction and to 

 the homology of the germ layers. 



The first subject has been most fully treated by von Kennel 

 (10) and Lang (13). 



In a comprehensive survey of the whole question they arrived 

 at conclusions to which I have been led by the study of his- 

 tological details in a more limited field. In general they con- 

 sider the capacity for regeneration, which is found so widely 

 distributed in the animal kingdom, to be the starting-point for 

 the more special case of budding. 



From my observations on the Oligochaeta the two processes 

 seem to be connected on definite structural grounds, as shown 

 by the following series of forms in the order in which they 

 stand : — 



Lumbricus, Tubifex, Lumbriculus, Nais. 



In Lumbricus the process is slowest, the non-differentiated 

 cells appear to be all alike, and the elements of old and of new 

 tissue are confused beyond distinction until one has found the 

 key elsewhere. 



Tubifex shows a great advance upon this condition, and per- 

 haps the greatest gap in the series is to be found here. In 

 Tubifex the presence of neoblasts shows a well-marked adapta- 



